When your house isn't watertight, and neither is the cover

It's every householder's nightmare. Early last year, the ground floor of Kevin and Adrienne Loftus's Cheshire home started to sink. "The wooden and laminated floorboards had become uneven and cracks had appeared in some. We also found many of the floor tiles in the bathroom had broken," says Mr Loftus.

A structural surveyor inspec- ted the damage and gave a bleak warning: the ground floor would have to be taken up and relaid, and the family would need to be rehoused until the work was done.

Despite their concern, the couple believed their seven-year-old house was covered by a 10-year "Buildmark" warranty from the National House-Building Council (NHBC), the guarantor behind the developer that built their home.

But the NHBC told them this was not the case. "It said the problems were cosmetic rather than structural and therefore not covered by the [warranty]," says Mr Loftus.

Refusing to accept this, the couple sent off the report, which underlined that the problem was structural, to the NHBC.

But nothing was done until, months later, water began seeping through the walls and floor of their cellar.

This time, a call to the NHBC prompted a visit from the body's inspector, who confirmed that the house had a genuine structural problem - meaning they were covered by the warranty. Repair work began in December.

The Loftuses' story highlights the potential problems with seemingly watertight warranties.

New-build guarantees last 10 years and are issued by three companies: NHBC, Zurich Municipal and Premier Guarantee.

Minor, cosmetic defects like shoddy paintwork and damaged door handles are covered for the first two years. Over the next eight, policyholders are insured for bigger, structural flaws such as cracked walls and weak roofs. The two-tier system was set up because it can take longer for structural problems to emerge.

However, interpretations by the warranty provider of what constitutes a minor or major defect can leave homeowners in the lurch. And it's not just a question of lengthy delays before the work gets done.

"Developers tell owners that warranties are safety nets and proof that properties have been finished to the highest standards," says Catriona Bright, a partner at New Build Inspections, a company that examines new developments for defects. "But if you probe a little deeper, you'll find they have many caveats and exclusion clauses." These can include problems with central heating, smoke alarms, fences, waste disposal units, water-softening equipment and goods such as ovens and dishwashers.

One contributory factor is that buyers of new or nearly-new houses often don't bother with a full structural survey, reasoning that a modern house is unlikely to need one. But it's vital you do, says Ms Bright. "Pay a chartered building surveyor [typically £300 to £400] to inspect it before you complete, so you can get any problems put right before you move in." Ask your solicitor to include a "retention" clause in the contract, forcing the developer to do the work.

For anyone buying a new or nearly-new home, there are other steps to take too. First, check your developer is a member of a warranty scheme, and scrutinise the guarantee's exclusions.

Enquire about any other home (or non-residential) projects your developer has been involved in; a £4 fee to Companies House will make this information easier to come by.

And if it's a brand-new house, make regular visits during your home's construction. If nothing else, it may help keep the company on its toes.